The present invention relates in general to a circuit for minimizing radiation from another circuit. More particularly, there is described herein a circuit adapted for minimizing radiation from a clock circuit. The concepts of this invention are particularly applicable to such equipment as computing devices, calculators and the like.
In recent years there has been a substantial growth in the use of highly sophisticated computing equipment, both in the home and for industrial and commercial applications. This computing equipment, such as calculators, electronic games, computers, terminals, printers, etc., has become increasingly complex and now routinely uses computing rates exceeding 100,000 instructions per second. Along with this increase in speed has come an increased amount of unwanted electromagnetic radiation from these devices causing interference with radio and television communication. One of the objectives of the present invention is to provide a means which can be employed in combination with digital circuitry for reducing electromagnetic radiation from these computing devices. As described hereinafter, the source of this radiation is examined and then a means is devised as described herein for reducing the radiation from the source of radiation.
Circuits derive their name from the fact that in them electricity runs in loops through the components and often back into the power supply. Time varying currents in these circuits produce radiation in accordance with the following formula: ##EQU1## where: .omega.=frequency in radians
I.sub.o =current, amps PA0 .mu..sub.o =free space permeability PA0 .epsilon..sub.o =free space dielectric constant PA0 a=radius in meters PA0 E=electrical field in volts per meter PA0 r=measuring distance PA0 c=speed of light in meters per second
This formula represents the field generated by a small loop carrying sinusoidally varying current. At a given distance from the loop, the field strength is proportional to the magnitude of the current, the square of the loop radius, and the square of the frequency of radiation.
A computing device, such as a computer, electronic game, or calculator has hundreds of such loops. The signals in these loops are usually digital with a fundamental frequency of less than 100 Mhz. The loops that tend to radiate the most are the large radius loops which have signals that are periodic, have sharp edges and large current changes. The periodicity of the wave forms produce harmonics whose field strength, according to the above formula, increases with the square of the frequency. Both the radius of the loop and the magnitude of the current changes in the loop also effect the amount of radiation.
Of all the circuits in a computing device, the circuit with the highest frequency, sharpest edges and the highest current change is generally the clock circuit. The clock circuit furthermore is the heart of many digital devices. It provides the reference timing for the rest of the circuitry and often is also associated with the central processing unit of a computing device. Therefore, the clock circuit is one which needs to be designed in such a way so as to reduce or eliminate the radiation that is emanating from the circuit.
In comparison, most other circuits in the computing devices do not produce as much radiation as the clock driver. Also, these other circuits often conduct only random, uncorrelated signals that tend to produce broad band radiation with very low energy in relatively low interference potential. The clock driver, on the other hand, produces high energy, temporally correlated signals which have great interference potential. Furthermore, these signals are often distributed along many leads producing large amounts of total radiation.
Accordingly, one object of the present invention is to provide a circuit used in association with another circuit such as a clock circuit for reducing the radiation therefrom. As indicated previously this is particularly of concern in computing devices where a great many of these circuits are employed.
Another object of the present invention is to provide a circuit in accordance with the preceding object and which is relatively simple in construction.